Safe to put V1a coupling capacitor on a switch for variety?

[Chuck H]

I've actually put the switch on the floor with a low gain amp (18W type) and it worked fine. There's a jack socket on the rear panel and I used a 10ft guitar lead to a metal box footswitch. Inside the footswitch there was also a LED and 9V battery connected to the other pole of the switch so you could see when it was in boost mode.

Absolutely! In fact I have a mid boost design that does what you propose. I haven't used it yet because I haven't run across a situation where it could be put seamlessly into a design. But it's there on the shelf and ready when I want it.

I'm starting a new thread this morning in the mods/tweaks forum that proposes something similar for the reverb switch on ab763 type circuits to reduce noise.

So yeah, cathodes can handle some lead length and can be forgiving with lead routing.

[Mr. Bill]

Nice Chuck!

Here's a rough schematic without power as of yet. I know I could do the cathode switches more correctly to make them functional at all times, and I'm well aware this is overkill. I do want to clarify 2 things though. This is a tinkering amp that will be used in a recording studio by many guitar players. It will never leave my room. I plan to use it to further hear combinations on the classic circuits that I like or don't like with input from other musicians. I am aware that I will have to switch some of these with the amp off or on standby and that it could be laid out better......but, these are the parts I have on hand and the wife just may murder me in my sleep if she sees me on Mouser's website again within the next few weeks.

The only thing missing here is the NFB cap switch, which I've done before and is no biggie.

Also, putting these on the back panel let's me add a gain stage without drilling or making a new board. This thing is so tacky already I might just paint it red with white chicken head knobs, ha!

[Dave H]

So earlier in the thread I mentioned the possibility of making the plate resistor on V1a switchable. Is this not advisable? Again, this is just a prototype that will never leave the room, so it would only be switched while off or on standby. If it's safe to do so, what rating switch would I need? I'm just trying to save myself from having to solder and unsolder parts in while I experiment a little.

If you must do it I'd use a plastic rotary switch like the one below. A safe way to switch between say a 100k and 220k plate resistor would be to connect a 100k and 120k in series. Connect the free end of the 100k to the B+ node, connect the free end of the 120k to the plate and put the switch across the 120k. Make the 100k 1W or more and it won't matter if the switch shorts to chassis as the current will be limited to a few mA by the 100k.

[Mr. Bill]

Dave, I think I've decided against that at this point. Plus I'd have to drill for it and I just want to get started. Pretty easy to just tack that in and do a comparison and pick one.

[Chuck H]

If you must do it I'd use a plastic rotary switch like the one below. A safe way to switch between say a 100k and 220k plate resistor would be to connect a 100k and 120k in series. Connect the free end of the 100k to the B+ node, connect the free end of the 120k to the plate and put the switch across the 120k. Make the 100k 1W or more and it won't matter if the switch shorts to chassis as the current will be limited to a few mA by the 100k.]

Smart and eloquent. I like it

[Mr. Bill]

Here's the current progress on the actual chassis layout. I'm not sure about the voltage divider after the first stage, might want to put that on a pot. Also undecided on a third value for stage 2. I don't want to just leave it open, so I'm open to ideas.

Also, is the only thing I have to do to make the cathode switches functional live is strap a high value resistor from input to ground?

Anybody see anything here that could get me into trouble? Thanks for everything.



Current schematic a few posts up.

[Mr. Bill]

Would someone mind verifying for me that this is the correct wiring for the cliff jacks to do what I want them to do? I would like for the first jack to hit the extra gain stage I'm adding, and the second jack to be The stock 2203 "high" input for the rest of the amp. Thanks again and again.

[Chuck H]

Big kudos. That looks like it should work, assuming...

That the switch arms are from the left (duh, but just being sure) and that you have a proper circuit (coupling cap and hopefully a voltage divider/pot) between stages one and two.

I don't know if you just worked that out for yourself or if you got it from the web, but that arrangement looks like it even grounds the front stage output when the stock amp jack is used. Very nice. One stage in front won't always bleed, but sometimes it can depending on a few things. That extra insurance grounding it is a lot better than letting it float.

[Mr. Bill]

Thanks Chuck. I looked at the original 2203 layout and then looked at the cliff jack spec sheet to understand how they work better. Then I modified that to give myself 2 high Independence inputs that would lift the ground on the second jack with the first jack inserted and ground the extra stage when the second jack was plugged in. I have a new respect for cliff jacks now. Deceivingly complicated, haha.

[Chuck H]

I've had to work out a couple of switch diagrams that nearly popped a vein in my head. Sometimes it helps me to draw the switches in schematic form so I can see the function without having to imagine it. I do this in ink. Then I work in pencil so I can temporarily draw in or erase contact in action for each switch function (or each jack) with the actual traces explicitly shown.

[Mr. Bill]

Ha! I did a similar thing. I drew versions of the switches in each combination and then routed wires around on them with pencil.

[Chuck H]

Not your first rodeo then You're going to have fun with this. Just keep playing your guitar and don't fall into the trap that some of us have suffered. The one where you spend too much time obsessing about the sound and not enough time being a playing musician. Remember, any amp can sound good in the right hands. Because the right hands find the musical qualities of whatever they play through.

[Mr. Bill]

Oh I am absolutely obsessed! I mentioned this before, but I barely even play guitar. I'm a drummer and recording engineer by trade. I just really like to build me own stuff. I've made some compressors, solder all my own cables, pads, and filters, etc. I've been getting really into tubes lately and it's quite fun indeed!

While I'm picking your brains on here....is there a "best practice" for AC heater wiring? I was reading on the valve wizard site, and that guy is quite adamant about the heater wires maintaining the twist right up to the pins and actually passing over the socket base. It looks really clean, but seems like it would be difficult, especially around the cathode follower. I know most people twist and break out in a circle around the socket. Is there much benefit of trying to have it pass through the middle like that?

[Chuck H]

I think the idea is that you want the inductive advantage of the twisting to be maintained where it matters most. Right near the tender grid leads. IMHE you can ease up a little if it helps with the soldering. Because of the higher current (and therefor larger lead gauge) filaments are enough of a PITA without trying to have the twist break for the shortest imaginable (or barely manageable) untwist. I usually do go with a hard twist right to over the sockets, but I have also done a more relaxed build where I branched off the twist in the aforementioned hoops. But that wasn't a higher gain, cascade preamp. It was a vintage style amp. So very likely less sensitive to hum/noise. Ideally you do twist right to the pins if you can. and I usually do. It also helps to fully bypass the first gain stage cathode since that's the worst place to allow any hum into the signal chain. Sometimes it also helps to DC elevate the AC filament circuit. Basically, the center tap of the winding, or false center tap (a pair of resistors when no center tap is provided or used) would be referenced to a DC voltage of some 25VDC or more. I won't go into the theory behind this, but if all grounding is ideal elevating the filaments is the next level in hum rejection. But ideally any modern high gain build uses regulated DC filaments because that is the surest way to avoid hum entering the signal chain through the filament and cathode circuits. I haven't built an uber gain amp since before the DC filament thing was fashionable. So I used a DC elevated AC circuit and did just fine. YMMV

[Mr. Bill]

We have liftoff! My 3 year old was pretty excited to help me tape off the ground points and paint the chassis. I think he's more excited than I am. Finish turned out great so far.